Asking for "hydroxyapatite" does not specify a material. The name identifies a mineral phase, not the behaviour it will show in your matrix. Two powders that pass the same identity report can behave very differently in a suspension, in a polymer or along a sintering route.
A useful specification answers three questions for every attribute: what value, within what band, and measured how. Without all three, what exists is an expectation, not a specification.
Identity: phase and composition
The first block describes what the material is. For calcium phosphates that means crystalline phases by XRD, chemical composition and the Ca/P ratio — close to 1.67 for stoichiometric hydroxyapatite.
Industry standards show the level of detail a complete identity requires. ISO 13779-3 defines the protocol for phase purity, crystallinity ratio, Ca/P ratio and trace metals. ASTM F1185, for medical-grade hydroxyapatite, requires a minimum of 95% hydroxyapatite by quantitative XRD and sets limits on the minor phases — α-TCP, β-TCP, tetracalcium phosphate and free calcium oxide — as well as limits for arsenic, cadmium, mercury and lead.
Not every project needs medical grade. But every project needs to know which minor phases are tolerable, because those are usually what explains unexpected behaviour.
Form: particle size, morphology and surface
The second block describes how the material presents itself. Particle size at three points (D10, D50, D90), morphology by SEM and, when interaction with the medium matters, surface area by BET.
This is where most incomplete specifications fail. A D50 on its own does not describe a distribution: two powders with the same D50 and different widths behave differently in packing, in dispersion and in reactivity.
To specify is to choose a band, not a target. A single value with no tolerance hands the supplier a decision that belongs to the project: how much variation the application can absorb.
The band, and why it is the most important item
No synthesis route delivers the same number twice. What a good specification declares is the window within which the result is still acceptable, and that window comes from the application — not from the process.
Setting the band early avoids both expensive errors. Too tight, and it rejects batches that would have worked, raising cost with no technical gain. Too wide, and it accepts batches that will fail later, where fixing the problem costs more.
The report is part of the specification
An attribute with no declared test method is not verifiable. Particle size by laser diffraction and by sieving do not produce the same number for the same powder; quantitative XRD depends on the refinement procedure. So the specification records the method alongside the value, and each batch report states the result obtained by the declared method.
At Triplet, the specification is the product of the technical conversation that precedes synthesis. It sets identity, form, bands and methods before the first batch exists, and it is what each batch is characterised and documented against — which turns batch-to-batch comparison into a verification rather than an impression.
References
- ISO 13779-3:2018 — Implants for surgery — Hydroxyapatite — Part 3: Chemical analysis and characterization of crystallinity ratio and phase purity
- ASTM F1185-23 — Standard Specification for Composition of Medical-Grade Hydroxylapatite for Surgical Implants
- ISO 13320:2020 — Particle size analysis — Laser diffraction methods
- ICH Q8(R2) — Pharmaceutical Development, on critical quality attributes and design ranges
